Portable rechargeable touch sensor equipped devices have recently proliferated. Such devices include, by way of non-limitive example, smartphones, tablet computers, remote controllers, gaming controllers, and laptop computers. In these touch sensor equipped devices, the touch sensor is usually transparent and overlies a display to form a touch screen. In a laptop computer, a touch sensor has been used in the touchpad and is more recently used in a touch screen as well. While there are many other touch sensor technologies including acoustic, optoelectronic, and resistive, at present capacitive touch sensors are most commonly used in rechargeable touch sensor equipped devices.
In a capacitive touch sensor, a high frequency signal is successively applied to each of a set of conductive areas of the touch sensor. Contact or proximity of a person's finger to one of the conductive areas establishes or enhances a capacitive circuit which affects the amplitude of the high frequency signal applied to the conductive area. By sensing changes in the amplitude or the current of the high frequency signal, the person's touch may be inferred, which is to say sensed.
External battery chargers commonly used for rechargeable touch sensor devices, such as tablet computers and smartphones, generally use switched mode power supplies. In a switched mode power supply, a circuit through a primary side of a transformer is interrupted by a switch operated at high frequency thereby inducing a current in the secondary side of the transformer. Compared to traditional linear power supplies, switched mode power supplies are lighter and less expensive; however, they produce high frequency noise.
It is often the case that touch screen devices are used while coupled to a charger. While the charger is coupled, it will often generate a noise spectrum that overlaps with the fundamental frequency or higher frequency components of the above mentioned high frequency signal used by the capacitive sensor. The noise spectrum produced by the charger creates background noise distributed over the touch sensor effectively reducing the sensitivity of the touch sensor. To make matters worse the battery charge controller, which is a module internal to the rechargeable device, demands different amounts of current from the external battery charger depending on the charge state of the batteries of the rechargeable device, and in order to provide such variable amounts of current the external battery charger will vary either a switching frequency or a pulse width of a switched power signal and either of these changes will alter the above mentioned noise spectrum generated by the external charger, such that one cannot rely on contending with a fixed noise spectrum.
Furthermore, many different external chargers, produced by different companies and of different design are available for many rechargeable touch sensor equipped devices, and generally speaking each different external charger will produce a different noise spectrum that may vary in a different manner as a function of battery charge state.
For the designer of the rechargeable touch sensor equipped device (e.g., smartphone, tablet computer, remote controller, gaming controller, laptop computer) the variability of the noise spectrum produced by external chargers makes it difficult to design a touch sensor that is immune to such noise. Thus, there is an opportunity for a rechargeable touch sensor equipped device in which the touch sensor is less susceptible to noise generated by external chargers.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present disclosure.